Compact waveguide-coax transition

ABSTRACT

A compact waveguide-to-coaxial transition coupling having a half-height waveguide with a shorted termination and with a tapered ridge waveguide section having a port therein one-quarter wavelength from the termination into which port is &#39;&#39;&#39;&#39;floatingly&#39;&#39;&#39;&#39; fitted a coaxial line termination with impedance matching steps thereon, and inductive buttons on the side walls of the half-height waveguide laterally of the tapered ridge waveguide section to provide transition of microwave at a voltage standing wave ratio of 1.1 or less over at least a 13 percent band.

United States Patent 91 [451 v Apr, 3, 1973 Princeton, NJ.

[73] Assignee: The United States of America as represented by the Secretary of the Navy [22] Filed: June 20, 1972 [21] Appl. N0.: 264,671

[52] US. Cl. ..333/21 R, 333/34, 333/35,

[51] Int. Cl. ..'......H0lp 5/10, H03h 7/38 [58] Field of Search ..333/21 R, 21 A, 32-35, 333/98 R, 98 TN, 97 R [56] References Cited UNITED STATES PATENTS 2,633,293" "3/1953" Cohn..... ..333/34 Primary Examiner-Rudolph U. Rolinec Assistant Examiner-Marvin Nussbaum Attorney-R. S. Sciascia et al.

[57] ABSTRACT A compact waveguide-to-coaxial transition coupling having a half-height waveguide with a shorted termination and with a tapered ridge waveguide section having a port therein one-quarter wavelength from the termination into which port is floatingly fitted a coaxial line termination with impedance matching steps thereon, and inductive buttons on the side walls of the half-height waveguide laterally of the tapered ridge waveguide section to provide transition of microwave at a voltage standing wave ratio of l.l or less over at least a 13 percent band.

8 Claims, 4 Drawing Figures I l 2 :I' 2 .63)

Borghetti ..333/98 X PATENT1IU1P113 191s 1 ,7 5, 4

Wfiko 1 2 SM CH 1.10 VSWR I ON 25 51 COAX Y LINE WITH 105 5001015 MINATED COMPACT WAVEGUIDE-COAX TRANSITION BACKGROUND OF THE INVENTION This invention relates to a coupling from a waveguide to a coaxial cable and more particularly to the coupling of a half-height waveguide section at right angles to a 25 ohm coaxial line having power handling capacity, moderate impedance bandwidth, and noncontacting coaxial inner conductor with the waveguide area to provide the transformer action desired with the proper impedance match. While these known transition devices have been successful in operation,,there were complications in impedancematching requiring implementation with materials of substantial bulk and weight.

SUMMARY OF THE INVENTION In the present invention a halfheight waveguide section can be used to provide small size. A tapered ridge member is positioned within the waveguide backed by a short section of shorted waveguide. An opening in the long sidewalls of the waveguide through the thick end '-'of the tapered ridge member accommodates the inner conductor of the coaxial line which floats in the tapered ridge member opening. This condition of a non-contacting inner conductor is satisfied by the use of a quarter-wave, open circuited stub in series at the junction of the waveguide and coaxial line. A metal button on opposite side walls of the waveguide inthe area of the tapered ridge member improves the impedance over the desired bandwidth. The inner conductor of the coaxial line at the junction has impedance matching steps as is typical for such couplings. It is accordingly a general object of this invention to provide a compact waveguide-to-coaxial line transition coupling that has a floating junction to simplify assembly and is capable of producing a voltage standing wave ratio (VSWR) of 1.1 or less over at least a 13 percent band.

BRIEF DESCRIPTION OF-THE DRAWING These and other objects and the attendant advantages, features, and uses will become more apparent to those skilled in the art as a more detailed description proceeds when taken in consideration with the accompanying drawing in which:

FIG. I is a partly elevational and partly cross-sectional view of the preferred embodiment of the invention;

FIG. 2 is a cross-sectional view of the preferred embodiment of the invention taken along the line A-A'of FIG. 1;

FIG. 3 is a view of a Smith chart showing the measured impedance in the shaded area for the transition coupling of this invention; and

FIG. 4 presents a plot of the measured VSWR characteristic of the transition over at least 13 percent band above and below the center frequency.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring particularly to FIGS. 1 and 2 a half-height waveguide section 10 has a coaxial leg input 11 and an open-circuited stub 12. Dimensions of the waveguide section are given in wavelength of the microwaves used therein at mid band frequency. The waveguide section is 0.188 of a wavelength (ho) high, 0.8 )to wide, and the coaxial leg 11 at least 0.5 )to long but preferably 0.63 A0. The section 10 ends in a shorted termination 13 that is one-quarter wave distant from the center of the leg 11.

' Within the waveguide section 10 is a tapered ridge waveguide member 15 of metal suitably attached to the underneath side of one long sidewall having a tapered surface 16 facing the open waveguide. The large end near the shorted termination 13 has a hole 17 concentrically aligned with the leg 11 and the series stub 12. The length of the taper 16 is at least a half wave but preferably 0.63 A0 similar or equal 'in length to the coaxial leg 11. Also within the waveguide section 10 are two metal buttons 18, each suitably affixed to the inner side walls in the area adjacent to the center of the tapered portion 16 of the tapered ridge member 15 to improve the impedance bandwidth.

A coaxial cable 20 having an inner conductor 21 enters through the coaxial leg 11 with a terminating stub portion 22 concentrically supported in the hole 17 in the tapered ridge member 15. The coaxial cable may be supported in any suitableway but can be secured by a thin Teflon sleeve 23 to facilitate assembly and disassembly of the coupling. The coaxialline has stepped changes in diameter over the length of the inner conductor to provide the best impedance match for a 25- ohm coaxial line over the wavelength band. The exact dimensions of these stepped sections are determined experimentally after the ridged waveguide section is constructed,'the typical change being 0.145 A0.

The impedance of the matched waveguide is in the order of ohms. The characteristic impedance of the coaxial line is 25 ohms, or a relatively high impedance change of 7.2:1 over the 13 percent frequency band. However, both the waveguide leg and the coaxial line leg are at least one-half wavelength long. Thus, there is room for four cascaded quarter-wave transformers (or equivalent circuitry) between the transition input and output ports. With the proper choice'of characteristic impedances for the four transformer sections, the maximum VSWR on any one transformer is only about 1.64:1, and broad impedance bandwidth is assured.

The tapered ridge member 15 functions as the first two transformer sections. The width of the tapered ridge member 15 is chosen to provide the maximum waveguide loading. The ridge extends over the maximum length available to minimize the ridge taper angle 16 and reduce the impedance discontinuities. The maximum height of the tapered ridge member 15 at the junction 17,22 is chosen as a reasonable compromise between the needs of impedance transformation and voltage breakdown considerations at the junction regron.

Referring more particularly to FIG. 3, the impedance was measured over the required band on the 25 ohm coaxial line relative to the junction 17,22. All impedance points fell within the small shaded region shown on the Smith chart.

The inner conductor length of the open-circuited series stub 12 is adjusted to present a short circuit at its input. The characteristic impedance of this stub 12 is chosen to be small in the order of 4.3 ohms so that the voltage at the open circuited end is low, and also to minimize the reactance change over the band. The voltage at the open end of 12 at mid-frequency is equal to the product of the line characteristic impedance and the line current at the base of the stub. For the assumed peak power under the worst condition, this voltage is only a few hundred volts. The base reactance change over the 13 percent band is less than 1 ohm.

Referring more particularly to FIG. 4 there is plotted in graphic form the VSWR for a ohm coaxial line with the waveguide section terminated. The plot provides the VSWR along the abscissa and the frequency right and left of the center frequency of 1.00 along the ordinate axis. Thus it may be seen that the VSWR is less than 1.1 over at least 13 percent band of frequen- While modification and changes may readily occur said waveguide with said opening in alignmentwith said series and coupling stubs with said taper rising from said waveguide wall in a direction toward said shorting termination;

metal button on opposite side walls of said waveguide along the taper of said tapered ridge member; and coaxial line entering through said coupling stub with a junction end concentrically supported floatingly within said tapered ridge member opening, said coaxial line within said coupling stud having impedance matching steps therein whereby 'a transition of microwave energy in said waveguide to said coaxial line and vice versa are accomplished with high power capability, with a voltage standing wave ratio of not over 1.1 over at least a 13 percent band, and with a half-height waveguide to a predetermined low impedance matching coaxial line providing a compact transition coupling. 2. A compact waveguide-to-coaxial line transition coupling as set forth in claim 1 wherein said series and coaxial coupling stubs are centered one quarter wavelength from said shorted termination and said junction end of said coaxial line is one-quarter wavelength. 3. A compact waveguide-to-coaxial line transition coupling as set forth in claim 2 wherein said tapered ridge member 1s from 0.5 to 0.63 of a wavelength from the point of taper to the centerline of said opening and its width is 0.28 of a wavelength. 4. A compact waveguide-to-coaxial line transition coupling as set forth in claim '3 wherein said waveguide coupling section isabout 0.188 of a wavelength high and about 0.8 of a wavelength wide. 5. A compact waveguide-to-coaxial line transition coupling as set forth in claim 4 wherein saidv metal buttons are each' about 0.14 of a wavelength in diameter. I 6. A compact waveguide-to-coaxial line transition coupling as set forth in claim 5 wherein said impedance matching steps are each about 0.145

of a wavelength. 7. A compact waveguide-to-coaxial line transition coupling as set forth in claim 6 wherein said concentric supporting of said coaxial line junction end is by a thin wall Teflon sleeve. 8. A compact waveguide-to-coaxial line transition coupling as set forth in claim 7 wherein said open-circuited series stub is a quarter-wave in series with said junction. 

1. A compact waveguide-to-coaxial line transition coupling comprising: a section of 1/2 height waveguide for a predetermined microwave frequency ending in a shorted termination with an open circuited series stub and a coaxial line coupling stub in alignment in opposite walls thereof; a tapered ridge member having an opening therethrough on the thick end thereof mounted in said waveguide with said opening in alignment with said series and coupling stubs with said taper rising from said waveguide wall in a direction toward said shorting termination; a metal button on opposite side walls of said waveguide along the taper of said tapered ridge member; and a coaxial line entering through said coupling stub with a junction end concentrically supported floatingly within said tapered ridge member opening, said coaxial line within said coupling stud having impedance matching steps therein whereby a transition of microwave energy in said waveguide to said coaxial line and vice versa are accomplished with high power capability, with a voltage standing wave ratio of not over 1.1 over at least a 13 percent band, and with a half-height waveguide to a predetermined low impedance matching coaxial line providing a compact transition coupling.
 2. A compact waveguide-to-coaxial line transition coupling as set forth in claim 1 wherein said series and coaxial coupling stubs are centered one quarter wavelength from said shorted termination and said junction end of said coaxial line is one-quarter wavelength.
 3. A compact waveguide-to-coaxial line transition coupling as set forth in claim 2 wherein said tapered ridge member is from 0.5 to 0.63 of a wavelength from the point of taper to the centerline of said opening and its width is 0.28 of a wavelength.
 4. A compact waveguide-to-coaxial line transition coupling as set forth in claim 3 wherein said waveguide coupling section is about 0.188 of a wavelength high and about 0.8 of a wavelength wide.
 5. A compact waveguide-to-coaxial line transition coupling as set forth in claim 4 wherein said metal buttons are each about 0.14 of a wavelength in diameter.
 6. A compact waveguide-to-coaxial line transition coupling as set forth in claim 5 wherein said impedance matching steps are each about 0.145 of a wavelength.
 7. A compact waveguide-to-coaxial line transition coupling as set forth in claim 6 wherein said concentric supporting of said coaxial line junction end is by a thin wall ''''Teflon'''' sleeve.
 8. A compact waveguide-to-coaxial line transition coupling as set forth in claim 7 wherein said open-circuited series stub is a quarter-wave in series with said junction. 